Lithium metal has many industrial uses including, e.g., employing a blanket of the liquid metal for breeding purposes in nuclear fusion reactors in the nuclear power industry. Additionally, lithium metal is used in lightweight, compact primary and secondary lithium batteries for military and commercial applications and as a degasifier in the production of high-conductivity copper and bronze. Another use of this metal is in the synthesis of organometallic compounds for applications in the fields of rubber, plastics and medicines. Lithium metal is generally produced for such uses by electrolysis of an eutectic mixture of highly pure molten lithium chloride and potassium chloride.
Naturally occurring brines found, e.g., in the United States and Chile, contain reasonable concentrations of lithium, in the form of lithium chloride. These brines are viable reserves for lithium recovery. These brines also contain varying amounts of boron, calcium and other components. Some typical components of naturally occurring brines are identified in the Table below entitled "Saline Brine Analyses".
TABLE __________________________________________________________________________ SALINE BRINE ANALYSES Weight Percent Great Geothermal Silver Atacama Dead Sea Salt Lake Bonneville Salton Sea Peak Brine Ocean Israel Utah Utah California Nevada Chile __________________________________________________________________________ Na 1.05 3.0 7.0 9.4 5.71 6.2 7.17 K 0.038 0.6 0.4 0.6 1.42 0.8 1.85 Mg 0.123 4.0 0.8 0.4 0.028 0.02 0.96 Li 0.0001 0.002 0.006 0.007 0.022 0.02 0.15 Ca 0.040 0.3 0.03 0.12 2.62 0.02 0.031 SO.sub.4 0.25 0.05 1.5 0.5 0.00 0.71 1.46 Cl 1.900 16.0 14.0 16.0 15.06 10.06 16.04 Br 0.0065 0.4 0.0 0.0 0.0 0.002 0.005 B 0.0004 0.003 0.007 0.007 0.039 0.005 0.04 Li/Mg 1/12720 1/2000 1/135 1/60 1/1.3 1/1 1/6 Li/K 1/3800 1/300 1/70 1/90 1/71 1/20 1/12 Li/Ca 1/400 1/150 1/5 1/17 1/119 1/1 1/0.2 Li/B 1/4 1/1.5 1/1.2 1/1 1/1.8 1/0.25 1/0.27 __________________________________________________________________________
Some of these brines have high concentrations of lithium and a low magnesium to lithium ratio, generally about 1:1 to 6:1, which allow for a simplified process of concentrating, purifying and recovering lithium chloride brine. Lithium carbonate is then obtained by treatment of the brine with soda ash.
In a well-known method for preparing lithium metal, the lithium carbonate is converted to lithium hydroxide via a liming process. The lithium hydroxide is then converted to lithium chloride by treatment with hydrochloric acid followed by drying. All of these steps are utilized to obtain lithium chloride of sufficient purity for use in the electrolytic production of lithium metal from lithium chloride.
The impurities, such as, magnesium, calcium, sodium, sulfate and boron present in lithium containing natural brines, should be minimized to produce a lithium chloride product suitable for production of lithium metal by electrolysis. Alkali and alkaline earth metals, such as sodium, magnesium and calcium, must be substantially removed, otherwise, they will report as contaminants in the lithium metal. Simple technical means for their removal from the metal are not cost effective.
During electrolysis of lithium chloride, non-volatile anions will accumulate, resulting in rapid short-circuiting of the cell. For example, boron normally contained in lithium carbonate made from brines has a negative impact on subsequent use of the lithium carbonate for many applications. The boron content is particularly detrimental to lithium chloride produced from lithium carbonate for lithium metal production. The non-volatile borate anion in the electrolytic cell accumulates as deposits in the cell and thereby increases resistivity and decreases current efficiency in the cell, resulting in premature shut down of the cell. These losses in efficiency and premature shut down undesirably increase the cost of lithium metal production. Desirably, boron concentrations in electrolytic cells should be 25 ppm or less of boron, equivalent to about 100 ppm or less of borate ion.
As presently practiced in the industry, boron is removed from, or substantially reduced in, lithium chloride brine on a commercial basis by first converting the lithium chloride brine containing substantial impurities into lithium carbonate via a process of precipitation of lithium carbonate with soda ash. The lithium carbonate is subsequently converted to lithium hydroxide by lime treatment of lithium carbonate to produce lithium hydroxide and waste calcium carbonate. Crystallization of the lithium hydroxide substantially removes the boron and alkali metal impurities by way of a bleed stream. The lithium hydroxide is then treated with hydrochloric acid to produce lithium chloride or treated with CO.sub.2 to produce high purity lithium carbonate. These conversions are accomplished according to the following series of reactions: EQU LiCl+Na.sub.2 CO.sub.3 .fwdarw.NaCl+Li.sub.2 CO.sub.3 1 EQU Li.sub.2 CO.sub.3 +Ca(OH).sub.2 +2H.sub.2 O.fwdarw.2LiOH.H.sub.2 O+CaCO.sub.3 2 EQU LiOH.H.sub.2 O+HCL.fwdarw.LiCl+2H.sub.2 O 3 EQU 2 LiOH.H.sub.2 O+CO.sub.2 .fwdarw.Li.sub.2 CO.sub.3 +3H.sub.2 O4
Lithium carbonate crystals precipitated from lithium chloride brines containing boron typically retain a contaminating quantity of lithium borate. For example, purified lithium containing brines produced in Nevada, U.S.A. and in Chile contain approximately 7,000 ppm of lithium and approximately 2,000 ppm of boron. Lithium carbonate produced from such brines normally contains about 0.04% boron, which exists as approximately 0.2% lithium borate. Therefore, precipitation of lithium carbonate is not an adequate means by which boron can be excluded from the resultant lithium salt.
A number of additional methods for boron removal have been used in the field of lithium metal manufacture. Among such methods include treatment of a brine with slaked lime to precipitate calcium borate and/or, where brines contain substantial magnesium impurities, magnesium borate. Attempts to absorb borates on clays, on HCO.sub.3 -- and Cl type resins, or on activated alumina in the presence of magnesium have also been employed to reduce the boron content of brines. Another unsatisfactory method is precipitating borate as a borophosphate concentrate by treating the brine with lime in combination with phosphoric acid. Brines have also been acidified to precipitate boric acid, and treated by solvent-solvent extraction, i.e. with n-butanol. None of these methods have proven to be cost effective for widespread commercial application.
As an example of efforts extant in the art to remove boron from lithium-containing brines, U.S. Pat. No. 4,261,960 discloses the removal of boron, as well as magnesium and sulfate, by treatment of the brine with an aqueous slurry of slaked lime and an aqueous solution of calcium chloride, followed by concentrating. U.S. Pat. No. 3,885,392 discloses the extraction of boron from magnesium chloride solutions which contain no lithium by contacting the solution with a solution, in petroleum ether, of a fatty alcohol like iso-octyl alcohol.
In addition, other methods for removal of borates and boric acids from brines of Searles Lake are described in D. S. Arnold, "Process Control in Boric Acid Extraction", Metallurgical Society Conferences, Vol. 49, 125-140, Gordon and Breach, Science Publishers, New York, (1968) [Hydrometallurgy Session, Second Annual Operating Conference, The Metallurgical Society of AIME, Philadelphia, Pa., Dec. 5-8, (1966)]. See, also, D. S. Arnold, "A New Process for the Production of Boric Acid", 19th Annual Technical Meeting, South Texas Section of AIChE, Galveston, Tex. (Oct. 23, 1964); C. R. Havighorst, "Kirkpatrick Award Winner/AP&CC's New Process Separates Borates from Ore by Extraction", Chemical Engineering, 70, 228-232 (Nov. 11, 1963. In addition other publications and U.S. patents on boric acid extraction date from the early 60's, including U.S. Pat. Nos. 2,969,275; 3,111,383; 3,297,737; 3,336,115; 3,370,093; 3,436,344; 3,479,294 and 4,261,961.
Applying known methods of boron removal to the concentrated brines of Silver Peak, Nev. which contain only 0.6% lithium, for example, requires almost 40 pounds of brine for production of one pound of lithium carbonate. In addition, these brines are not saturated in lithium chloride and therefore result in substantial solubilities of extractant, providing an additional economic penalty.
Although lithium brines are normally the source for low cost lithium carbonate, the resulting lithium chloride obtained from the lithium carbonate from brine has not been satisfactory for the production of lithium metal by way of electrolysis due to the presence of the borate ion.
Thus, although there are many means for boron removal from lithium-containing brines, none to date are commercially satisfactory for producing low boron lithium carbonate because of technological difficulty, inadequate purity of end product, or excessive cost. There remains, therefore, a need in the art for a satisfactory method for production of a boron-free lithium carbonate compound from lithium-containing brines.